Differences

This shows you the differences between two versions of the page.

--- linux:firewall [2008/04/29 11:08]
greebo
+++ linux:firewall [2019/04/15 10:18]
zagi
@@ Line 1: / Line 1: @@
-  #!/bin/bash
+[[linux:firewall6|Linux IPV6 firewall]]\\
-  echo "*************"
+[[linux:firewall_blocktor| how to block TOR network in realtime]]\\
-  echo "* Running $0"
+[[http://www.fs-security.com/|FS security]]\\
-  echo "*************"
+<code bash |>
+#!/bin/bash
+echo "*************"
+echo "* Running $0"
+echo "*************"
+echo "* http://tnt.aufbix.org/ linux firewall script"
+echo
+echo  "It was sad music. But it waved its sadness like a battle flag."
+echo  " It said the universe had done all it could, but you were still alive."
+echo
+echo "					Discworld"
+TNX_IDIOT="yes"
+echo " how iptables work in linux kernel"
+echo
+echo ">-[prerouting]->	+ >-[forward]->	+ >-[postrouting]->"
+echo "			|		|"
+echo "			[input] >--->[output]"
+# path to iptables and iproute2 files
+IPTB="/sbin/iptables"
+IP="/sbin/ip"
+# name of our Internet and intranet interfaces
+# use INTRANET="eth1+" or INTERNET="eth0+"
+# if you have more ifaces (example: eth0:0)  towards Intranet/Internet
+#
+# WAN Interface
+INTERNET="eth0"
+# ADSL - INTERNET="ppp0"
+#
+# LAN Interface
+INTRANET="eth1"
-  echo
+# what IPs are used in intranet
-  echo  "It was sad music. But it waved its sadness like a battle flag."
+LAN="192.168.6.0/24"
-  echo  " It said the universe had done all it could, but you were still alive."
-  echo
-  echo "					Discworld"
-  TNX_IDIOT="yes"
+# what is our static  IP (if we have one)
+GW_IP="X.X.X.X"
-  echo " how iptables work in linux kernel 2.4.x/2.6.x"
+# what TCP ports/services we allow (and FORWARD) from Internet
-  echo
+# use " " as delimiter
-  echo ">-[prerouting]->	+ >-[forward]->	+ >-[postrouting]->"
+TCP_PORTS="25 53 80"
-  echo "			|		|"
-  echo "			[input] >--->[output]"
+# what UDP ports/services we allow (and FORWARD) from Internet
+# use "," as delimiter
+UDP_PORTS="53,123"
+# which ports we forward into our intranet
+# use "," as delimiter
+FWD_TCP_PORTS="1214,6346"
-  # path to iptables and iproute2 files
+# set to 1 if we you have intranet
+WE_HAVE_INTRANET="0"
-  IPTB="/sbin/iptables"
-  IP="/sbin/ip"
+#
+TRUSTED_HOSTS="193.77.1.1/32 \
-  # name of our Internet and intranet interfaces
+.93.224.0/19 \
-  INTRANET="eth1"
+.18.32.0/24"
-  INTERNET="eth0"
-  # ADSL - INTERNET="ppp0"
+# enable IP forwarding (routing!)
+echo "0" > /proc/sys/net/ipv4/ip_forward
-  # what IPs are used in intranet
-  LAN="192.168.6.0/24"
+# enable PMTU (mss/mtu discovery)
+echo "1" > /proc/sys/net/ipv4/tcp_mtu_probing
-  # what is our static  IP (if we have one)
-  GW_IP="X.X.X.X"
+# first we flush the tables and policy
+$IPTB -F
-  # what TCP ports/services we allow (and FORWARD) from Internet
+$IPTB -X
-  # use " " as delimiter
+$IPTB -F INPUT
-  TCP_PORTS="22 25 53 80"
+$IPTB -F FORWARD
+$IPTB -F OUTPUT
-  # what UDP ports/services we allow (and FORWARD) from Internet
-  # use "," as delimiter
+$IPTB -t nat -F
-  UDP_PORTS="53"
+# new chain for SSH and HTTP access
-  # which ports we forward into our intranet
+$IPTB -N ssh-access
-  # use "," as delimiter
+$IPTB -N http-access
-  FWD_TCP_PORTS="1214,6346"
+# port redirection (transparent proxy)
-  # set to 1 if we you have intranet
+# redirect all outgoing traffic that is NOT for the GW to local (GW) ports
-  WE_HAVE_INTRANET="0"
+# DNS (53/tcp and 53/udp) and SMTP (25/tcp)
+#$IPTB -t nat -A PREROUTING -i ! $INTERNET -p tcp -s $LAN -d ! $LAN --dport 53 -j REDIRECT
-  echo "0" > /proc/sys/net/ipv4/ip_forward
+#$IPTB -t nat -A PREROUTING -i ! $INTERNET -p udp -s $LAN -d ! $LAN --dport 53 -j REDIRECT
+#$IPTB -t nat -A PREROUTING -i ! $INTERNET -p tcp -s $LAN -d ! $LAN --dport 25 -j REDIRECT --to-ports 25
-  # first we flush the tables and policy
-  $IPTB -F
+# INPUT TABLE
-  $IPTB -F INPUT
+$IPTB -P INPUT DROP
-  $IPTB -F FORWARD
-  $IPTB -F OUTPUT
+# statefull firewall makes most hits
-  $IPTB -t nat -F
+$IPTB -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-  # port redirection (transparent proxy)
+# move all SSH and HTTP traffic to apropriate chains
-  #$IPTB -t nat -A PREROUTING -i ! $INTERNET -p tcp -s $LAN -d ! $LAN --dport 53 -j REDIRECT
+$IPTB -A INPUT -p tcp -m state --syn --state NEW --dport 22 -j ssh-access
-  #$IPTB -t nat -A PREROUTING -i ! $INTERNET -p udp -s $LAN -d ! $LAN --dport 53 -j REDIRECT
+$IPTB -A INPUT -p tcp -m state --syn --state NEW --dport 80 -j http-access
-  #$IPTB -t nat -A PREROUTING -i ! $INTERNET -p tcp -s $LAN -d ! $LAN --dport 25 -j REDIRECT --to-ports 25
+# ssh chain
-  # INPUT TABLE
+for sshhostese in $TRUSTED_HOSTS;
-  $IPTB -P INPUT DROP
+        do
+        $IPTB -A ssh-access -s $sshhostese -j ACCEPT
-  # statefull firewall
+        done
-  $IPTB -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
+ # Connection limit for SSH connections (1 connection per minute PER source IP)
+ # - usefull against ssh scanners if you MUST open SSH for every IP!
-  # IPSEC
+$IPTB -A ssh-access -m hashlimit --hashlimit 1/minute --hashlimit-burst 1 --hashlimit-mode srcip --hashlimit-name ssh -j ACCEPT
-  #$IPTB -A INPUT -i $INTERNET -p udp --sport 500 --dport 500  -j ACCEPT
+$IPTB -A ssh-access -j DROP
-  #$IPTB -A INPUT -i $INTERNET -p 50 -j ACCEPT
+# ssh
-  #$IPTB -A INPUT -i $INTERNET -p 51 -j ACCEPT
+# http
-  # we allow all traffic from $INTRANET and localhost interfaces
+for httphostese in $TRUSTED_HOSTS;
-  $IPTB -A INPUT -i $INTRANET -j ACCEPT
+        do
-  $IPTB -A INPUT -i lo -j ACCEPT
+        $IPTB -A http-access -s $httphostese -j ACCEPT
+        done
-  #$IPTB -A INPUT -m state --state INVALID -m limit --limit 1/minute -j LOG --log-prefix "INVALID packet> "
+# http
-  #$IPTB -A INPUT -m state --state INVALID -j DROP
+# IPSEC
-  #
+#$IPTB -A INPUT -i $INTERNET -p udp --sport 500 --dport 500  -j ACCEPT
-  $IPTB -A INPUT -i $INTERNET -m pkttype --pkt-type broadcast -j DROP
+#$IPTB -A INPUT -i $INTERNET -p 50 -j ACCEPT
-  $IPTB -A INPUT -i $INTERNET -m pkttype --pkt-type multicast -j DROP
+#$IPTB -A INPUT -i $INTERNET -p 51 -j ACCEPT
-  #FIN is set and ACK is not
+# we allow all traffic from $INTRANET and localhost interfaces
-  $IPTB -A INPUT -p tcp --tcp-flags ACK,FIN FIN -j DROP
+$IPTB -A INPUT -i $INTRANET -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
-  $IPTB -A INPUT -p tcp --tcp-flags ACK,FIN FIN -j LOG --log-prefix "FIN> "
+$IPTB -A INPUT -i lo  -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
-  #PSH is set and ACK is not
+$IPTB -A INPUT -m state --state INVALID -m limit --limit 1/minute -j LOG --log-prefix "packet not in conntrack> "
-  $IPTB -A INPUT -p tcp --tcp-flags ACK,PSH PSH -j DROP
+$IPTB -A INPUT -m state --state INVALID -j DROP
-  $IPTB -A INPUT -p tcp --tcp-flags ACK,PSH PSH -j LOG --log-prefix "PSH> "
+#
-  #URG is set and ACK is not
+$IPTB -A INPUT -i $INTERNET -m pkttype --pkt-type broadcast -j DROP
-  $IPTB  -A INPUT -p tcp --tcp-flags ACK,URG URG -j DROP
+$IPTB -A INPUT -i $INTERNET -m pkttype --pkt-type multicast -j DROP
-  $IPTB  -A INPUT -p tcp --tcp-flags ACK,URG URG -j LOG --log-prefix "URG> "
+#FIN is set and ACK is not
-  # Block portscans:
+$IPTB -A INPUT -p tcp --tcp-flags ACK,FIN FIN -j LOG --log-prefix "FIN> "
-  $IPTB -A INPUT -p tcp --tcp-flags ALL ALL  -j LOG --log-prefix "XMAS scan> "
+$IPTB -A INPUT -p tcp --tcp-flags ACK,FIN FIN -j DROP
-  $IPTB -A INPUT -p tcp --tcp-flags ALL ALL  -j DROP
+#PSH is set and ACK is not
-  #no flag is set
+$IPTB -A INPUT -p tcp --tcp-flags ACK,PSH PSH -j LOG --log-prefix "PSH> "
-  $IPTB -A INPUT -p tcp --tcp-flags ALL NONE -j LOG --log-prefix "NULL scan> "
+$IPTB -A INPUT -p tcp --tcp-flags ACK,PSH PSH -j DROP
-  $IPTB -A INPUT -p tcp --tcp-flags ALL NONE -j DROP
+#URG is set and ACK is not
-  $IPTB -A INPUT -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j LOG --log-prefix "pscan> "
+$IPTB  -A INPUT -p tcp --tcp-flags ACK,URG URG -j LOG --log-prefix "URG> "
-  $IPTB -A INPUT -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP
+$IPTB  -A INPUT -p tcp --tcp-flags ACK,URG URG -j DROP
-  #SYN and FIN are both set
+# Block portscans:
-  $IPTB -A INPUT -p tcp --tcp-flags SYN,FIN SYN,FIN -j LOG --log-prefix "pscan 2> "
+$IPTB -A INPUT -p tcp --tcp-flags ALL ALL  -j LOG --log-prefix "XMAS scan> "
-  $IPTB -A INPUT -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
+$IPTB -A INPUT -p tcp --tcp-flags ALL ALL  -j DROP
-  #FIN and RST are both set
+#no flag is set
-  $IPTB -A INPUT -p tcp --tcp-flags FIN,RST FIN,RST -j DROP
+$IPTB -A INPUT -p tcp --tcp-flags ALL NONE -j LOG --log-prefix "NULL scan> "
-  $IPTB -A INPUT -p tcp --tcp-flags FIN,RST FIN,RST -j LOG --log-prefix "fin/rts flag>"
+$IPTB -A INPUT -p tcp --tcp-flags ALL NONE -j DROP
+$IPTB -A INPUT -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j LOG --log-prefix "pscan> "
-  $IPTB -A INPUT -f -j LOG --log-prefix "FRAGMENT> "
+$IPTB -A INPUT -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP
-  $IPTB -A INPUT -f -j DROP
+#SYN and FIN are both set
-  $IPTB -A INPUT -p tcp --tcp-flags ALL SYN,FIN -j LOG --log-prefix "SYNFIN-SCAN>"
+$IPTB -A INPUT -p tcp --tcp-flags SYN,FIN SYN,FIN -j LOG --log-prefix "pscan 2> "
-  $IPTB -A INPUT -p tcp --tcp-flags ALL SYN,FIN -j DROP
+$IPTB -A INPUT -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
-  $IPTB -A INPUT -p tcp --tcp-flags ALL URG,PSH,FIN -j LOG --log-prefix "NMAP-XMAS-SCAN>"
+#FIN and RST are both set
-  $IPTB -A INPUT -p tcp --tcp-flags ALL URG,PSH,FIN -j DROP
+$IPTB -A INPUT -p tcp --tcp-flags FIN,RST FIN,RST -j LOG --log-prefix "fin/rts flag>"
+$IPTB -A INPUT -p tcp --tcp-flags FIN,RST FIN,RST -j DROP
-  $IPTB -A INPUT -p tcp --tcp-flags ALL FIN -j LOG --log-prefix "FIN-SCAN>"
-  $IPTB -A INPUT -p tcp --tcp-flags ALL FIN -j DROP
+$IPTB -A INPUT -f -j LOG --log-prefix "Lost FRAGMENT> "
+$IPTB -A INPUT -f -j DROP
-  $IPTB -A INPUT -p tcp --tcp-flags ALL URG,PSH,SYN,FIN -j LOG --log-prefix "NMAP-ID>"
-  $IPTB -A INPUT -p tcp --tcp-flags ALL URG,PSH,SYN,FIN -j DROP
+$IPTB -A INPUT -p tcp --tcp-flags ALL SYN,FIN -j LOG --log-prefix "SYNFIN-SCAN>"
+$IPTB -A INPUT -p tcp --tcp-flags ALL SYN,FIN -j DROP
-  #SYN and RST are both set
-  $IPTB -A INPUT -p tcp --tcp-flags SYN,RST SYN,RST -j LOG --log-prefix "SYN-RST> "
+$IPTB -A INPUT -p tcp --tcp-flags ALL URG,PSH,FIN -j LOG --log-prefix "NMAP-XMAS-SCAN>"
-  $IPTB -A INPUT -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
+$IPTB -A INPUT -p tcp --tcp-flags ALL URG,PSH,FIN -j DROP
-  # Connection limit for SSH connections ( 1 connection per minute)
+$IPTB -A INPUT -p tcp --tcp-flags ALL FIN -j LOG --log-prefix "FIN-SCAN>"
-  $IPTB -A INPUT -p tcp -m state --syn --state NEW --dport 22 -m limit --limit 1/minute --limit-burst 1 -j ACCEPT
+$IPTB -A INPUT -p tcp --tcp-flags ALL FIN -j DROP
-  $IPTB -A INPUT -p tcp -m state --syn --state NEW --dport 22 -j DROP
+$IPTB -A INPUT -p tcp --tcp-flags ALL URG,PSH,SYN,FIN -j LOG --log-prefix "NMAP-ID>"
-  # what we allow from Internet
+$IPTB -A INPUT -p tcp --tcp-flags ALL URG,PSH,SYN,FIN -j DROP
-  for i in $TCP_PORTS
+#SYN and RST are both set
+$IPTB -A INPUT -p tcp --tcp-flags SYN,RST SYN,RST -j LOG --log-prefix "SYN-RST> "
+$IPTB -A INPUT -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
+# what we allow from Internet - TCP ports
+for i in $TCP_PORTS
 	do
-		$IPTB -A INPUT -p tcp -m state --syn --state NEW  --dport $i -j ACCEPT
+	$IPTB -A INPUT -p tcp -m state --syn --state NEW  --dport $i -j ACCEPT
-    done
+        done
-  $IPTB -A INPUT -p udp -m multiport --dport $UDP_PORTS -j ACCEPT
+# what we allow from Internet - UDP ports
+$IPTB -A INPUT -p udp -m multiport --dport $UDP_PORTS -j ACCEPT
-  # identd requests
-  $IPTB -A INPUT -p tcp --dport 113 -j REJECT --reject-with tcp-reset
+# identd requests
+$IPTB -A INPUT -p tcp --dport 113 -j REJECT --reject-with tcp-reset
-  # traceroute
-  $IPTB -A INPUT -p udp -m limit --limit 3/second  --sport 32769:65535 --dport 33434:33523 -j ACCEPT
+# traceroute (udp - IOS, Uni*es)
+$IPTB -A INPUT -p udp -m limit --limit 3/second  --sport 32769:65535 --dport 33434:33523 -j ACCEPT
-  # Log and drop ICMP fragments (shouldn't happen at all, but often used for DoS)
-  $IPTB -A INPUT -i $INTERNET --fragment -p icmp -j LOG --log-prefix "Fragmented incoming ICMP> "
+# Log and drop ICMP fragments (shouldn not happen at all, but often used for DoS)
-  $IPTB -A INPUT -i $INTERNET --fragment -p icmp -j DROP
+$IPTB -A INPUT -i $INTERNET --fragment -p icmp -j LOG --log-prefix "Fragmented incoming ICMP> "
+$IPTB -A INPUT -i $INTERNET --fragment -p icmp -m hashlimit --hashlimit 10/second --hashlimit-burst 1 --hashlimit-mode srcip --hashlimit-name icmp-frag -j ACCEPT
-  # thou shall NOT block ALL ICMP, but only allow usefull ICMP types to pass trough
-  $IPTB -A INPUT -p icmp --icmp-type 0  -m limit --limit 30/second -j ACCEPT
+# thou shall NOT block ALL ICMP, but only allow usefull ICMP types to pass trough
-  $IPTB -A INPUT -p icmp --icmp-type 3  -m limit --limit 30/second -j ACCEPT
+# echo-reply
-  $IPTB -A INPUT -p icmp --icmp-type 4  -m limit --limit 30/second -j ACCEPT
+#$IPTB -A INPUT -p icmp --icmp-type 0 -m hashlimit --hashlimit 10/second --hashlimit-burst 1 --hashlimit-mode srcip --hashlimit-name icmp0 -j ACCEPT
-  $IPTB -A INPUT -p icmp --icmp-type 11 -m limit --limit 30/second -j ACCEPT
+# unreachables
-  $IPTB -A INPUT -p icmp --icmp-type 12 -m limit --limit 30/second -j ACCEPT
+$IPTB -A INPUT -p icmp --icmp-type 3 -m hashlimit --hashlimit 10/second --hashlimit-burst 1 --hashlimit-mode srcip --hashlimit-name icmp3 -j ACCEPT
-  #icmp-traceroute
+# source-quench (depreciated)
-  $IPTB -A INPUT -p icmp --icmp-type 30 -m limit --limit 30/second -j ACCEPT
+#$IPTB -A INPUT -p icmp --icmp-type 4 -m hashlimit --hashlimit 10/second --hashlimit-burst 1 --hashlimit-mode srcip --hashlimit-name icmp4 -j ACCEPT
+# timeout (forward loop prevention)
-  # echo-request
+$IPTB -A INPUT -p icmp --icmp-type 11 -m hashlimit --hashlimit 10/second --hashlimit-burst 1 --hashlimit-mode srcip --hashlimit-name icmp11 -j ACCEPT
-  $IPTB -A INPUT -p icmp --icmp-type 8  -m limit --limit 3/second -j ACCEPT
+# parameter problem
+$IPTB -A INPUT -p icmp --icmp-type 12 -m hashlimit --hashlimit 10/second --hashlimit-burst 1 --hashlimit-mode srcip --hashlimit-name icmp12 -j ACCEPT
-  # if the default policy is not DROP then we must use this
+#icmp-traceroute
-  #$IPTB -A INPUT -p icmp -j DROP
+$IPTB -A INPUT -p icmp --icmp-type 30 -m hashlimit --hashlimit 10/second --hashlimit-burst 1 --hashlimit-mode srcip --hashlimit-name icmp30 -j ACCEPT
+# echo-request
-  # FORWARD TABLE
+$IPTB -A INPUT -p icmp --icmp-type 8 -m hashlimit --hashlimit 10/second --hashlimit-burst 1 --hashlimit-mode srcip --hashlimit-name icmp8 -j ACCEPT
-  $IPTB -P FORWARD DROP
+# if the default policy is not DROP then we must use this
-  # port forwarding
+#$IPTB -A INPUT -p icmp -j DROP
-  #$IPTB	-A FORWARD	-p tcp	-i $INTERNET	-m multiport --dport $FWD_TCP_PORTS	-j ACCEPT
-  # START	/ port forwarding
-  # list forwarder ports in separate command lines
-  #$IPTB	-t nat	-A PREROUTING	-p tcp	-i $INTERNET --dport 1214  -j DNAT --to 192.168.1.10
-  #$IPTB	-t nat	-A PREROUTING	-p tcp	-i $INTERNET --dport 6346  -j DNAT --to 192.168.1.10
-  # END 	/ port forwarding
-  # statefull firewall
-  #$IPTB -A FORWARD -m state --state INVALID -j LOG --log-prefix "INVALID: "
-  $IPTB -A FORWARD -m state --state INVALID -j DROP
-  $IPTB -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
-  $IPTB -A FORWARD -m state --state NEW -i ! $INTERNET -j ACCEPT
-  $IPTB -A FORWARD -m pkttype --pkt-type broadcast -j DROP
-  $IPTB -A FORWARD -m pkttype --pkt-type multicast -j DROP
-  # NAT (IP masquerading)
-  #$IPTB -t nat -A POSTROUTING -o $INTERNET -j MASQUERADE
-  # NAT but to certain IP (if we have multiple Internet IPs)
+# FORWARD TABLE
-  $IPTB -t nat -A POSTROUTING -o $INTERNET -s $LAN -j SNAT --to-source $GW_IP
+$IPTB -P FORWARD DROP
-  # adsl
+# port forwarding
-  #$IPTB -I FORWARD --protocol tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
+#$IPTB	-A FORWARD	-p tcp	-i $INTERNET	-m multiport --dport $FWD_TCP_PORTS	-j ACCEPT
-  $IPTB -I FORWARD -o $INTERNET -p tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1400:1536 -j TCPMSS --clamp-mss-to-pmtu
+# START	/ port forwarding
+# list forwarder ports in separate command lines
+#$IPTB	-t nat	-A PREROUTING	-p tcp	-i $INTERNET --dport 1214  -j DNAT --to 192.168.1.10
+#$IPTB	-t nat	-A PREROUTING	-p tcp	-i $INTERNET --dport 6346  -j DNAT --to 192.168.1.10
+# END 	/ port forwarding
+# statefull firewall
+#$IPTB -A FORWARD -m state --state INVALID -j LOG --log-prefix "INVALID: "
+$IPTB -A FORWARD -m state --state INVALID -j DROP
+$IPTB -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
+$IPTB -A FORWARD -m state --state NEW ! -i $INTERNET -j ACCEPT
+$IPTB -A FORWARD -m pkttype --pkt-type broadcast -j DROP
+$IPTB -A FORWARD -m pkttype --pkt-type multicast -j DROP
-  # we allow only access to network cards (NIC) that have their MAC addresses listed
+# NAT (IP masquerading)
-  # in "valid-macs" file
+#$IPTB -t nat -A POSTROUTING -o $INTERNET -j MASQUERADE
-  #for mac in `cat valid-macs`; do $IPTB -I FORWARD -m mac --mac-source $mac -j fwfilter ; done
+# NAT but to certain IP (if we have multiple Internet IPs)
+$IPTB -t nat -A POSTROUTING -o $INTERNET -s $LAN -j SNAT --to-source $GW_IP
+# ADSL (PPPoE connections)
+#$IPTB -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
+$IPTB -I FORWARD -o $INTERNET -p tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1400:1536 -j TCPMSS --clamp-mss-to-pmtu
+# we allow only access to network cards (NIC) that have their MAC addresses listed
+# in "valid-macs" file
+#for mac in `cat valid-macs`; do $IPTB -I FORWARD -m mac --mac-source $mac -j fwfilter ; done
+# OUTPUT
+$IPTB -P OUTPUT DROP
+# only allow NEW and related connections out
+$IPTB -A OUTPUT -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
-  # list the rules
+# list the rules
-  $IPTB -L -v -n
+$IPTB -L -v -n --line
+$IPTB -t nat -L -v -n --line
-  echo $WE_HAVE_INTRANET > /proc/sys/net/ipv4/ip_forward
+echo $WE_HAVE_INTRANET > /proc/sys/net/ipv4/ip_forward
+</code>